Desensitizing solution for lithographic printing plate

ABSTRACT

A novel desensitizing solution is disclosed which is applied to the surface of a lithographic printing plate coated with a photosensitive layer of a photopolymerizable composition, the desensitizing solution contains a hydrophilic colloid, a free radical polymerization inhibitor and water.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a desensitizing solution applied to asensitized lithographic plate having a layer of photopolymerizablecomposition.

2. Description of the Prior Art

Among many types of currently available sensitized lithographic printingmaterials, a lithographic printing plate using a photopolymerizablecomposition as a photosensitive layer is known to have a long runningimpression and therefore can be used for printing a large number ofsheets. One example of the photopolymer conventionally applied to alithographic plate is a photosensitive material which mainly consists ofa photodimerizable sensitive resin, for instance, a combination of apolyester of p-phenylene diacrylate and 1,4-cyclohexane diol (describedin Belgian Pat. No. 696,533), or a reaction product of phenoxy resin andcinnamic acid (described in U.S. Pat. No. 3,387,976), and apolymerizable compound having an ethylenic unsaturated bond, and abinder. Examples of the combination of the ethylenic unsaturatedcompound with binder are described in U.S. Pat. No. 3,043,805, whichinclude esters such as diethylene glycol diacrylate, triethylene glycoldimethacrylate, and pentaerythritol triacrylate (as the ethylenicunsaturated compound) plus resins such as methylmethacrylate/methacrylic acid copolymer and styrene/itaconic acidcopolymer (as the binder). A photosensitive lithographic printingmaterial coated with such as photopolymerizable material is imagewiseexposed through a negative pattern and developed by a developer such asan organic solvent or an aqueous alkali which dissolves and removes theunexposed area.

Recently, a dry processable recording material has been proposed as analternative to the above-described printing material that is processedby a liquid developer. Japanese Patent Publication Nos. 9663/63 and22901/68 (U.S. Pat. No. 3,353,955) as well as Japanese PatentApplication (OPI) Nos. 7728/72 (U.S. Pat. No. 3,770,438) (The term "OPI"as used herein refers to a "published unexamined Japanese patentapplication") and 46315/75 describe such a new recording material whichmakes use of the photopolymerization of a photopolymer and the resultingchange in adhesive strength. To be more specific, this type of recordingmaterial comprises a plastic, metal or paper substrate which is coatedwith a layer of photosensitive composition containing a polymer asbinder and an initiator of photopolymerization of unsaturated monomer,said layer being further overlaid with a cover of a thin transparentfilm. After imagewise exposure with the original placed on the cover,the cover is peeled off to have the exposed area (or unexposed area) ofthe photosensitive layer left on the substrate and the unexposed area(or exposed area) left on the cover so that a negative and a positiveimage (or positive and negative image) are formed on the substrate andcover, respectively. A photosensitive lithographic printing plate usinga photopolymerizable composition in such peelable photosensitivematerial is described in Japanese Patent Application (OPI) No. 9501/77and Japanese Patent Application No. 66353/76.

A lithographic printing plate is usually subjected to desensitizationfor rendering the non-image area hydrophilic. Desensitization (1)completely removes the photosensitive layer which was not removed bydevelopment, (2) enhances the hydrophilicity of the non-image area bychemical treatment of the metal surface, and (3) further enhances thehydrophilicity of the non-image area by adsorption of hydrophiliccolloidal particles. Functions (1) and (2) are accomplished by cronaktreatment, phosphate treatment, nital treatment or zirconium salttreatment. Function (3) is generally achieved by coating with gumarabic. To prevent reaction between the photosensitive layer and thesubstrate, a presensitized plate is generally subjected to anodizationof the aluminum substrate or chemical treatment with zirconium fluoride,or with sodium silicate as described in U.S. Pat. No. 2,714,066. Suchtreatments are effected prior to coating of a photosensitive solution,and after development, an aqueous solution mainly consisting of gumarabic is used to gum the plate for desensitization.

While the primary purpose of gumming is to protect the hydrophilicity ofthe non-image area, it also has the effect of preventing sebum secretedby fingers, other greasy matter and dust from staining the preparedplate during storage until its use for printing as well as preventingoxidation from occurring during temporary shutdown of a printingmachine.

In recent years, there has been a demand in the printing industry for aprinting plate of longer running impression. For this purpose, theentire surface of a lithographic printing plate having aphotpolymerizable sensitive layer is irradiated with actinic rays afterits imagewise exposure and development. Generally referred to aspost-exposure or re-exposure, this treatment promotes the polymerizationof the image area until it is completely hardened.

However, with a lithographic printing plate that makes use of thephotopolymerization of a photosensitive layer which is to be developedby a liquid developer, it is impossible to remove the unhardened areacompletely by development. In addition, if the developer has been usedto process a number of printing plates, the photopolymerizable compoundreleased will be adsorbed on the non-image area and, upon post-exposure,will be polymerized to give a stained non-image area during printing.Likewise, development of a printing plate that uses a peelable covercauses a trace of photopolymerizable composition to be left on thenon-image area, and this composition will be polymerized uponpost-exposure to give a stained non-image area during printing.

SUMMARY OF THE INVENTION

As a result of various studies to overcome such defects of theconventional technique, a desensitizing solution has been found whichcan be applied to a lithographic printing plate having aphotopolymerizable composition without any chance of spoiling thenon-image area even if the plate is subjected to post-exposure of itsentire surface after image-wise exposure and development.

It is therefore a primary object of this invention to provide adesensitizing solution which is applicable to a lithographic printingplate having a photopolymerizable composition without staining thenon-image area even if it is subjected to post-exposure or post-heatingafter making the plate.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a desensitizing solution which contains apolymerization inhibitor and is applied to a lithographic printing platehaving an image formed thereof by photopolymerization.

The desensitizing solution for a lithograhic printing plate of thisinvention comprises a hydrophilic colloidal substance, a free radicalpolymerization inhibitor, and water.

The hydrophilic colloidal substance which is the first ingredient of thedesensitizing solution of this invention acts as a binder for theingredients in the desensitizing solution other than water. It also hasthe effect of rendering the lithographic printing plate hydrophilic. Inaddition, it has weak affinity for both an oil-based ink receptiveprinting image and oil-based printing ink and as soon as printing startsit is capable of removing solid ingredients in the desensitizingsolution from the printing image.

A suitable hydrophilic colloidal substance can be selected from thepolymer compounds described in Hydrophilic polymers, ed. by MataoNakamura, Kagaku Kogyo-Sha (1973), Tokyo; Water-Soluble Resins, ed. byR. L. Davidson and M. Sitting, Van Nostrand Reinhold (1968), New York;and Japanese Patent Publication No. 5093/60. Specific examples includegum arabic, gelatin, cellulose derivatives such as methyl cellulose,ethyl cellulose, carboxymethyl cellulose, carboxyethyl cellulose, ethylhydroxyethyl cellulose, ethyl methyl cellulose, hydroxymethyl cellulose,hydroxyethyl cellulose, hydroxydipropyl cellulose, hydroxyethylmethylcellulose, hydroxypropylmethyl cellulose, sodium carboxymethylcellulose, sodium carboxymethylhydroxyethyl cellulose, sodium cellulosesulfate, etc., dextrin, shellac, alginates, polyvinyl pyrrolidone,polyvinyl alcohol or derivatives thereof, polyacrylamide or copolymersthereof, acrylic acid copolymers, vinyl methyl ethyl/maleic anhydridecopolymer, vinyl acetate/maleic anhydride copolymer, styrene/ maleicanhydride copolymer and the like. These polymer compounds may be usedindependently or as a mixture. Of these colloids, gum arabic ispreferred. The water-soluble colloidal substance is contained in thedesensitizing solution of this invention in an amount in the range offrom about 0.1 to about 20 wt%, preferably from about 0.4 wt% to about15 wt%, based on the total weight of the ingredients of the solution.

The free radical polymerization inhibitor which is the secondcharacteristic ingredient of the desensitizing solution prevents theresidual polymerizable compound on the non-image area from geingpolymerized by post-exposure or post-heating, thus preventing theformation of a stained non-image area during printing. Examples of thefree radical polymerization inhibitor are quinone compounds, aminecompounds, nitro compounds, nitroso compounds, nitron compounds,phenols, disulfides, hydrazines, mercaptans, thioethers, dithiocarbamicacids, sulfene amides, phosphates, phosphites, phosphines and borates.More specific examples indlude hydroquinone, p-benzoquinone,chlorobenzoquinone, 2,5-dichloro-p-benzoquinone,2,6-dichloro-p-benzoquinone, 2,3-dimethyl-p-benzoquinone,methoxy-p-benzoquinone, methyl-p-benzoquinone, o,m,p-phenylenediamine,phenyl-β-naphthylamine, diphenylamine, N-N'-diphenyl-p-phenylenediamine,p-hydroxydiphenylamine, o,m,p-dinitrobenzene, 2,4-dinitrotoluene,1,3,5-trinitrobenzene, 1,3,5-trinitroanisole, 1,3,5-trinitrotoluene,dinitrodurene, o,m,p-nitrophenol, 2,4-dinitrophenol,o,m,p-nitrophenylcarboxylic acid, o,m,p-nitroaniline,2,4,6-trinitrophenol, nitrosobenzene, methyl-α-nitrosoisopropyl ketone,phenyl-t-butylnitron, pyrogallol, catechol, α- or β-naphthol, tertiarybutyl catechol, tertiary butyl phenol, 2,4,6-trimethylphenol, cresol,o,m,p-isopropylphenol, 2-amino-4-methylphenol, tetramethylthiuramdisulfide, 2,2-benzoylaminodiphenyl disulfide, 2-mercaptoimidazole,2-mercaptobenzimidazole, zinc diethyl dithiocarbamate, N-oxydiethylenebenzotriazole, 2-sulfene amide, tri-n-butyl phosphine, triphenylphosphite, trimethyl phosphate, and triphenyl borate. These and otherpolymerization inhibitors are disclosed in Polymer Handbook, ed. byImmergut and Brandrup, John Wiley & Sons, 2nd ed., pp II-53-55. Theinhibitors may be used independently or as a mixture. The polymerizationinhibitor is contained in the desensitizing solution of this inventionin an amount in the range of from about 0.001 to 20 wt%, preferably from0.5 to 5 wt%, based on the total weight of the ingredients of thesolution.

The third ingredient of the desensitizing solution of this invention iswater, which functions as the solvent of the solution. Suitable water isdistilled water, deionized water, water from which solid matter has beenremoved by filtration, tap water or the like. Since water is used tofill the portion of the desensitizing solution that is left unoccupiedby the other ingredients, its content is not to be limited to anyspecific range, but preferably it is contained in an amount in the rangeof from about 40 to 90 wt% based on the total weight of the ingredientsof the solution.

The desensitizing solution of this invention may optionally containadditional ingredients, for example, acidic compounds such as phosphoricacid, nitric acid, sulfuric acid, chromic acid and tannic acid, or basiccompounds such as sodium silicate, sodium metasilicate, potassiumsilicate and potassium hydroxide. These additional ingredients have theeffect of rendering the non-image area of a lithographic printing platehydrophilic.

The desensitizing solution of this invention preferably contains one ormore compounds selected from the group consisting of a nonionicsurfactant and a polyhydroxyalkane. When applied to a lithographicprinting plate, the desensitizing solution containing these compoundsspreads well, suitably controls the drying, maintains for a desiredperiod the effect of rendering the non-image area hydrophilic, gives astrain-free non-image area, and as soon as printing starts, thedesensitizing solution has its ingredients other than water easilyremoved from the printing image by being carried in oil-based printingink. The nonionic surfactant or polyhydroxyalkane used in this inventionis a water-soluble compound which is preferably hygroscopic. Specificexamples of such compound are polyalkylene glycols, polyethylene glycolalkyl ethers, polyethylene glycol alkylphenyl ethers, polyethyleneglycol esters, polyalkanols containing therein three or more hydroxygroups, phosphoric acid esters of alkanols, and phosphoric acid estersof monohydroxyethers.

Examples of the polyalkylene glycol are compounds of the formulaHO--C_(z) H_(2z) O)_(a) H (wherein z is an integer of from 1 to 6, and ais an integer of from 1 to about 500) such as ethylene glycol, propyleneglycol, butylene glycol, pentane diol, hexylene glycol, trimethyleneglycol, tetramethylene glycol, hexamethylene glycol, diethylene glycol,triethylene glycol, tetraethylene glycol, dipropylene glycol,tripropylene glycol, and polyethylene glycol. Examples of thepolyethylene glycol alkyl ether include polyethylene glycol alkylmonoether and diether which have a degree of polymerization of ethyleneglycol in the range of from about 5 to about 30 and wherein the alkylgroup has 12 to 25 carbon atoms. Examples of the polyethylene glycolalkylphenyl ether include polyethylene glycol alkylphenyl monoether anddiether which have a degree of polymerization of ethylene glycol in therange of from about 5 to about 30 and wherein the alkylphenyl group is aphenyl group substituted with an alkyl group having 1 to 12 carbonatoms. Specific examples are p-(6-methylheptyl)phenyl ether andnonylphenyl ether of polyethylene glycol, and octylphenoxypolyethoxyethanol. Examples of the polyethylene glycol esterinclude polyethylene glycol monoester and diester which have a degree ofpolymerization of ethylene glycol in the range of from about 5 to about30 wherein the ester-forming aliphatic acid is a saturated aliphaticacid having 12 to 30 carbon toms. Examples of the polyalkanol containingtherein three or more hydroxy groups include glycerin, sorbitol,pentaerythritol, diglycerin, dipentaerythritol, and tripentaerythritol.Examples of the phosphoric acid esters of alkanols are hexanolphosphate, octanol phosphate and decanol phosphate. Specific examples ofthe phosphoric acid esters of monohydroxyethers are 2-octyloxyethanolphosphate and 2-decyloxyethanol phosphate. Polyalkanols containingtherein three or more hydroxy groups or polyalkylene glycols arepreferred. Particularly preferred are ethylene glycol, diethyleneglycol, propylene glycol, hexamethylene glycol, glycerin, sorbitol andpentaerythritol.

The nonionic surfactant or polyhydroxyalkane illustrated above iscontained in the desensitizing solution of this invention in an amountin the range of from about 3 to about 30 wt%, preferably from about 7 toabout 20 wt%, based on the total weight of the ingredients of thesolution.

The desensitizing solution of this invention may further contain anorganic solvent to promote the penetration of the desensitizing solutioninto the lithograhic printing plate as well as to release the residualphotosensitive composition from the non-image area. Any type of organicsolvent can be used if it dissolves or swells the photosensitivecomposition. Specific examples of the organic solvent include glycolethers such as 2-methoxyethanol, 2-ethoxyethanol, 2-butoxyethanol and2-phenoxyethanol; ketones such as methyl ethyl ketone, methyl isobutylketone and diacetone alcohol; esters such as butyl acetate, isoamylacetate and n-amyl acetate; and alcohols such as butyl alcohol andbenzyl alcohol. The organic solvent illustrated above is contained inthe desensitizing solution of this invention in an amount in the rangeof from about 0.1 to about 20 wt%, preferably from about 1 to about 10wt%, based on the total weight of the ingredients of the solution.

In addition to these ingredients, the desensitizing solution of thisinvention preferably contains an anionic surfactant to increase themiscibility of the solvent with water. Examples of the anionicsurfactant are fatty acid salts, salts of sulfuric acid esters of higheralcohols, salts of phosphoric acid esters of aliphatic alcohols,sulfonates of dibasic fatty acid esters, sulfonates of fatty acidamides, (C₁ -C₅) alkyl allyl sulfonates, alkyl esters of sulfoaliphaticacids, naphthalenesulfonic acid salt condensed with formaldehyde. Acondensate of naphthalenesulfonic acid and formalin is preferred. Theanionic surfactant is contained in the desensitizing solution of thisinvention in an amount in the range of from about 0.1 to about 20 wt%,preferably from about 1 to about 10 wt%, based on the total weight ofthe ingredients of the solution.

The desensitizing solution of this invention may contain an acetate toprevent staining of the non-image area. Examples of the acetate aresodium acatate, potassium acatate, calcium acetate, magnesium acetate,ammonium acetate, and zinc acetate. Sodium acetate and potassium acetateare preferred.

If desired, the desensitizing solution of this invention may furthercontain at least one compound selected from the group consisting ofmolybdic acid, boric acid, nitric acid, phosphoric acid, polyphosphoricacid and water-soluble alkali metal salts (e.g., Li, Na, K) and ammoniumsalts thereof. Such compound is added to prevent fouling of thedesensitizing solution as well as to enhance and maintain thehydrophilicity of the non-image area of the lithographic printing plate,etc. It should not react with the other ingredients of the desensitizingsolution to form a precipitate or gel. Experiments have demonstratedthat the above acids, water-soluble alkali metal salts or ammonium saltsthereof are free from such precipitation or gelation.

Specific examples of the compound added to achieve the above statedpurposes include salts of molybdic acid such as lithium molybdate,sodium molybdate, potassium molybdate, ammonium heptamolybdate ((NH₄)₆Mo₇ O₂₄.4H₂ O), sodium phosphomolybdate (Na₃ PO₄.12MoO₃), and ammoniumphosphomolybdate ((NH₄)₃ PO₄.12MoO₃ 3H₂ O); salts of boric acid such aslithium metaborate (LiBO₂.2H₂ O), lithium tetraborate (Li₂ B₄ O₇.5H₂ O),sodium metaborate (NaBO₂), sodium tetraborate (Na₂ B₄ O₇.10H₂ O), sodiumdecaborate (Na₂ B₁₀ O₁₆.10H₂ O), sodium perborate (NaBO₂.H₂ O₂.3H₂ O),sodium borate peroxyhydrate (Na₂ B₄ O₇.H₂ O₂.9H₂ O), sodium boroformate(NaH₂ BO₃.2HCOOH.2H₂ O), potassium metaborate (KBO₂), potassiumtetraborate (K₂ B₄ O₇.5H₂ O), ammonium tetraborate ((NH₄)₂ B₄ O₇.4H₂ O),and ammonium biborate ((NH₄)HB₄ O₇.3H₂ O); salts of phosphoric acid suchas sodium phosphate, sodium hydrogenphosphate, sodiumdihydrogenphosphate, potassium phosphate, potassium hydrogenphosphate,potassium dihydrogenphosphate, sodium pyrophosphate, potassiumpyrophosphate, sodium metaphosphate, potassium metaphosphate, sodiumpolymetaphosphate, potassium polymetaphosphate, sodium polyphosphate,and potassium polyphosphate; and salts of nitric acid such as lithiumnitrate, sodium nitrate and ammonium nitrate.

These acids, water-soluble alkali metal salts or ammonium salts thereofare contained in the desensitizing solution of this invention in anamount in the range of from about 0.01 to about 10 wt%, preferably fromabout 0.2 to about 5 wt%, based on the total weight of the ingredientsof the solution.

This invention will hereunder be described in greater detail byreference to the following Examples.

EXAMPLE 1

A mechanically grained 2S aluminum substrate for lithographic printingplate (aluminum alloy consisting of 99% aluminum, 0.6% magnesium andsilicon 0.4%) was immersed in a 2 wt% aqueous sodium hydroxide at 40° C.for a period of 1 minute to partially etch its surface. After washingwith water, the etched plate was immersed in a mixture of sulfuric acidand chromic acid for a period of about 1 minute to expose the surface ofpure aluminum, then immersed in 20% sulfuric acid at 30° C. to beanodized at a voltage of 15 V D.C. and a current density of 3A/dm².

The anodized plate was immersed in aqueous sodium molybdate (having aconcentration of 2.0 wt% and a temperature of 65° C.) for a period of 90seconds, and dried.

The following composition was dissolved in a mixture of 100 ml of methylethyl ketone and 20 ml of N,N-dimethylformamide to prepare a solution ofphotosensitive composition.

    ______________________________________                                        Chlorinated polyethylene (Superchlon                                                                  10 g                                                  CPE-907LTA, a product of Sanyo                                                Kokusaku Pulp Co., Ltd.                                                       Pentaerythritol trimethacrylate                                                                       10 g                                                  2-Methyl anthraquinone  0.2 g                                                 Hydroquinone            0.1 g                                                 Copper phthalocyanine dye                                                                             0.2 g                                                 ______________________________________                                    

(Superchlon CPE-907LTA is a polymer having a viscosity of about 90 cpsin 40% toluene at 25° C. and a chlorine content of about 69 wt%.)

The resulting solution was applied to the separately prepared aluminumsubstrate with a coating machine (whirler) until the photosensitivelayer was about 4 μm thick after removal of the solvent. The substratewas dried at 80° C. for a period of 7 minutes and laminated with a 12 μmthick polyethylene terephthalate film under pressure to thereby preparea sensitized lithographic printing plate. Seven more plates wereprepared in the same manner. Each plate was exposed through a negativefilm for a period of 17 seconds to a PS light of type S (a 2 kw metalhalide lamp manufactured by Fuji Photo Film Co., Ltd.) placed at adistance of 1 meter. Upon lapse of 17 seconds for exposure, thepolyethylene terephthalate film was peeled off the substrate upon whicha photoset positive image (for printing) was formed. The unhardened area(unexposed area) was removed by being carried in the polyethyleneterephthalate film.

The resulting eight lithographic plates were then treated with eightdifferent desensitizing solutions of the following formulations. Eightmore lithographic printing plates were prepared by transporting at 30mm/sec the above plates while the entire surface of each plate wasexposed to a 2 kw mercury lamp (ORC-AHH-2000/C, a product of Ohku Mfg.Co., Ltd.) placed at a distance of 15 cm which caused the surface of theplate to be illuminated with a light intensity of from about 1,000 toabout 1,450 μw/cm²).

    ______________________________________                                        Formulations of Desensitizing Solutions:                                      ______________________________________                                        Desensitizing Solution (1)                                                    Carboxymethyl cellulose (Celogen 7A,                                                                     10 g                                               a product of Dai-Ichi Kogyo Seiyaku                                           Co., Ltd.)                                                                    Sodium silicate JIS No.3 (a product of                                                                   75 g                                               The Nippon Chemical Industrial Co., Ltd.)                                     p-Nitrophenol              50 g                                               Glycerin                   150 g                                              Ethylene glycol monobutyl ether                                                                          50 g                                               Demol NL (a product of Kao-Atlas Co., Ltd.)                                                              50 g                                               Sodium molybdate           5 g                                                Water                      750 g                                              Desensitizing Solution (2)                                                    Sodium alginate            3 g                                                p-Nitrophenylacetic acid   60 g                                               Sodium metasilicate        90 g                                               Ethylene glycol            80 g                                               Diacetone alcohol          70 g                                               Demol NL                   50 g                                               Potassium acetate          5 g                                                Water                      700 g                                              Desensitizing Solution (3)                                                    Dextrin (a product of Kanto Chemical                                                                     10 g                                               Co., Inc.)                                                                    p-Aminophenol              50 g                                               Potassium metasilicate     100 g                                              Glycerin                   100 g                                              Butyl acetate              50 g                                               Perex NBL                  50 g                                               Sodium molybdate           5 g                                                Water                      600 g                                              Desensitizing Solution (4)                                                    Gum arabic                 100 g                                              Phosphoric acid (85 wt % aq. soln.)                                                                      40 g                                               Ammonium dichromate        80 g                                               o-Phenylenediamine         60 g                                               Water                      1,000 g                                            Desensitizing Solution (a) (control)                                          Polyvinyl pyrrolidone (a product of                                                                      15 g                                               Tokyo Kasei Kogyo K.K.)                                                       Sodium silicate JIS No. 3  90 g                                               Hexamethylene glycol       50 g                                               Glycerin                   50 g                                               Sodium dihydrogenphosphate 4 g                                                Water                      900 g                                              Desensitizing Solution (b) (control)                                          Carboxymethyl cellulose    10 g                                               Phosphoric acid (85 wt % aq. soln.)                                                                      40 g                                               Ammonium phosphate         30 g                                               Water                      1,000 g                                            Desensitizing Solution (c) (control)                                          Vinylmethyl ether/maleic anhydride                                                                       25 g                                               copolymer                                                                     Amine silicate silica sol (40 wt % aq.                                                                   145 g                                              soln.) (QAS-40, a product of Nissan                                           Chemical Industries, Ltd.)                                                    Ethylene glycol            50 g                                               Potassium nitrate          3 g                                                Water                      700 g                                              Desensitizing Solution (d) (control)                                          Carboxymethyl cellulose (Celogen 7A)                                                                     20 g                                               Sodium silicate JIS No. 3  100 g                                              Glycerin                   110 g                                              Sodium molybdate           4 g                                                Water                      750 g                                              ______________________________________                                    

The lithographic printing plates treated with these desensitizingsolutions were used for printing under the conditions set forth below.The printing plates treated with the desensitizing solutions (1) to (4)but not subjected to post-exposure were free from stains on thenon-image area and could print about 40 thousand sheets of paper.

Those printing plates which were subjected to postexposure aftertreatment with the desensitizing solutions (1) to (4) were not only freefrom stains on the non-image area but could print at least 150 thousandsheets of paper. On the other hand, when the controls (treated withdesensitizing solutions (a) to (d)) were subjected to post-exposure, thenon-image area of each plate was excessively spoiled, thus failing toproduce commercially acceptable printed matter.

    ______________________________________                                        Printing Conditions                                                           ______________________________________                                        Printing machine:                                                                          Heidel SORK-Z                                                    Printing ink:                                                                              Webking KOPN (a product of Toyo Ink                                           Mfg. Co., Ltd.)                                                  Damping water:                                                                             fresh water                                                      Printing speed:                                                                            7,500 sheets/hour                                                ______________________________________                                    

EXAMPLE 2

A photosensitive solution of the following formulation was applied totwo aluminum substrates of Example 1 with a coating machine (whirler)until the photosensitive layer was about 2 μm thick after removal of thesolvent. Each substrate was dried at 100° C. for a period of 2 minutes.

    ______________________________________                                        Trimethylol propane triacrylate                                                                         8     g                                             Acrylic acid/methylacrylate copolymer                                                                   10    g                                             (molar ratio 80:20)                                                           2-Methyl anthraquinone    0.1   g                                             Methyl ethyl ketone       60    ml                                            Methyl cellosolve acetate 60    ml                                            ______________________________________                                    

Each sensitized plate was exposed through a negative film for a periodof 60 seconds to a PS light of type S (a 2 kw metal halide lampmanufactured by Fuji Photo Film Col., Ltd.) placed at a distance of 1meter. Thereafter, the plates were developed in aqueous sodium hydroxide(4 g of NaOH in 1 liter of water), washed with water and treated withthe desensitizing solutions (4) and (b). Two more lithographic plateswere prepared by the same procedure as described above and transportedat 30 mm/sec while the entire surface of each plate was exposed to a 2kw mercury lamp (ORC-AHH-2000/c, a product of Ohku Mfg. Co., Ltd.)placed at a distance of 15 cm.

These lithographic plates were used for printing under the sameconditions used in Example 1. Whether subjected to post-exposure or not,the printing plate treated with the desensitizng solution (4) wasentirely free from stains on the non-image area. The plate which was notpost-exposed could print 20 thousand sheets of paper whereas thepost-exposed plate could print at least 100 thousand sheets of paper.Post-exposure of the plate treated with the desensitizing solution (b)formed stains on the non-image area, thus failing to producecommercially acceptable printed matter.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A desensitizing solution which is to be appliedto the surface of a lithographic printing plate coated with aphotosensitive layer composed of a photopolymerizable composition, saiddesensitizing solution containing a hydrophilic colloid, a free radicalpolymerization inhibitor which is not a developing agent for silverhalide photographic materials and is an organic compound selected fromthe group consisting of quinone compounds, nitro compounds, nitrosocompounds, nitron compounds, α- or β-naphthol, tertiary butyl catechol,tertiary butyl phenol, 2,4,6-trimethylphenol, cresol, o-, m- orp-isopropylphenol, 2-amino-4-methylphenol, disulfides, mercaptans,thioethers, dithiocarbamic acids, sulfene amides, phosphates,phosphites, phosphines, borates, phenyl-β-naphthylamine, diphenylamineand mixtures thereof in an amount sufficient to normally preventselectively the formation of stain on non-imaged areas during printing,and water.
 2. The desensitizing solution of claim 1, wherein saidhydrophilic colloid is selected from the group consisting of gum arabic,gelatin, cellulose derivatives, shellac, alginates, polyvinylpyrrolidone, polyvinyl alcohol or derivatives thereof, polyacrylamide orcopolymers thereof, acrylic acid copolymers, vinyl methyl ether/maleicanhydride copolymer, vinyl acetate/maleic anhydride copolymer, andstyrene/maleic anhydride copolymer.
 3. The desensitizing solution ofclaim 1, wherein said hydrophilic colloid is present in an amount ofabout 0.1 to 20 wt%.
 4. The desensitizing solution of claim 1, whereinsaid polymerization inhibitor is present in an amount of about 0.001 to20 wt%.
 5. The desensitizing solution of claim 1, wherein said solutionadditionally comprises a nonionic surfactant or a polyhydroxyalkane. 6.The desensitizing solution of claim 5, wherein said surfactant orpolyhydroxyalkane is present in an amount of about 3 to 30 wt%.
 7. Thedesensitizing solution of claim 1 additionally comprising an organicsolvent.
 8. The desensitizing solution of claim 7, wherein said solventis present in an amount of about 0.01 to 20 wt%.
 9. The desensitizingsolution of claim 1 additionally comprising an anionic surfactant. 10.The desensitizing solution of claim 9, wherein said anionic surfactantis present in an amount of about 0.1 to 20 wt%.
 11. The desensitizingsolution of claim 1 additionally comprising an acetate.
 12. Thedesensitizing solution of claim 1 additionally comprising at least onecompound selected from the group consisting of molybdic acid, boricacid, nitric acid, phosphoric acid, polyphosphoric acid andwater-soluble alkali metal salts and ammonium salts thereof.
 13. Thedesensitizing solution of claim 6, wherein said polyhydroxyalkane is apolyalkylene glycol, a polyethylene glycol alkyl ether, a polyethyleneglycol alkylphenyl ether, a polyethylene glycol ester, a polyalkanolcontaining therein three or more hydroxy groups, a phosphoric acid esterof an ankanol, or a phosphoric acid ester of a monohydroxy ether. 14.The desensitizing solution of claim 1, wherein said solutionadditionally comprises a polyalkylene glycol represented by the formulaHO--C_(z) H_(2z) O)_(a) H wherein z is an integer of 1 to 6 and a is aninteger of from 1 to about
 500. 15. The desensitizng solution of claim1, wherein said hydrophilic colloid is gum arabic.
 16. The desensitizingsolution of claim 1, wherein said free radical polymerization inhibitoris a nitro compound.
 17. The desensitizing solution of claim 1, whereinthe free radical polymerization inhibitor is selected from the groupconsisting of p-benzoquinone, chlorobenzoquinone,2,5-dichloro-p-benzoquinone, 2,6-dichloro-p-benzoquinone,2,3-dimethyl-p-benzoquinone, methoxy-p-benzoquinone,methyl-p-benzoquinone, phenyl-β-naphthylamine, diphenylamine, o,m orp-dinitrobenzene, 2,4-dinitrotoluene, 1,3,5-trinitrobenzene,1,3,5-trinitroanisole, 1,3,5-trinitrotoluene, dinitrodurene, o,m orp-nitrophenol, 2,4-dinitrophenol, o,m or p-nitrophenylcarboxylic acid,o,m or p-nitroaniline, 2,4,6-trinitrophenol, nitrosobenzene,methyl-α-nitrosoisopropyl ketone, phenyl-t-butylnitron, α- orβ-naphthol, tertiary butyl catechol, tertiary butyl phenol,2,4,6-trimethylphenol, cresol, o,m or p-isopropylphenol,2-amino-4-methylphenol, tetramethylthiuram disulfide,2,2-benzoylaminodiphenyl disulfide, 2-mercaptoimidazole,2-mercaptobenzimidazole, zinc diethyl dithiocarbamate, N-oxydiethylenebenzotriazole, 2-sulfene amine, tri-n-butyl phosphine, triphenylphosphite, trimethyl phosphate, p-nitrophenyl acetic acid, and triphenylborate.